Paint coverage formula: perimeter × height ÷ coverage per gallon. Surface texture, dark-to-light changes, and the two-coat factor that doubles your estimate.
Paint coverage is measured in square feet per gallon and represents the area a single gallon covers in one coat. According to manufacturer specifications from major brands such as Sherwin-Williams and Behr, a gallon of standard interior latex paint covers approximately 350 to 400 square feet per coat on a smooth, previously painted surface. Primer covers less, roughly 200 to 300 square feet per gallon, because it is formulated to penetrate and seal rather than simply coat the surface.
Coverage is always stated per coat, and most quality paint jobs require two coats for full color depth and a uniform finish. A single coat over a contrasting color often looks uneven, and manufacturer warranty coverage generally assumes two coats. This means the practical coverage of a single gallon across a finished two-coat project is closer to 175 to 200 square feet of wall.
The Paint Quality Institute notes that the first coat on bare or porous drywall absorbs more material, so new construction and first-time paint jobs consume 20 to 30 percent more paint than repainting an already sealed surface. Treating primer and first coats as a separate, higher-use step is the most common reason a quick estimate comes up short.
The core calculation is paintable area divided by coverage, multiplied by the number of coats. First determine the paintable area: multiply the room perimeter by the ceiling height, then subtract the area of doors and windows. A standard interior door is about 21 square feet (3 by 7 feet) and a typical window is about 15 square feet.
Gallons = ( Paintable area ÷ Coverage per gallon ) × Number of coats
Worked example: a 12 by 12 foot room with 8 foot ceilings has a perimeter of 48 feet, giving 48 × 8 = 384 square feet of wall. Subtract one door (21 square feet) and two windows (30 square feet) to get 333 paintable square feet. At 350 square feet per gallon and two coats: (333 ÷ 350) × 2 = 1.9 gallons, rounded up to 2 gallons for purchase.
| Surface Type | Coverage per Gallon (1 coat) |
|---|---|
| Smooth painted drywall | 350-400 sq ft |
| Bare drywall (unprimed) | 250-300 sq ft |
| Smooth plaster | 350-400 sq ft |
| Textured or stucco wall | 200-250 sq ft |
| Brick or concrete block | 200-250 sq ft |
| Wood siding | 250-300 sq ft |
Several variables move the real-world gallon count away from the textbook number. Surface texture is the largest factor: a smooth painted wall delivers full coverage, while rough stucco or unpainted brick can cut effective coverage to 200 to 250 square feet per gallon because the texture increases true surface area and absorbs material into its pores.
Color change matters as much as texture. Painting a light color over a dark one, or covering a bold accent wall, often requires a third coat or a tinted primer to prevent the old color from showing through. Painting a dark color over a light one is more forgiving and may allow a single coat with premium paint. The contrast between the old and new color, not the color itself, is what drives the extra coats.
Application method also changes usage. Airless sprayers apply paint far faster than a roller but waste 15 to 30 percent of the material to overspray, while a roller transfers most of the paint directly to the wall. Brush work is efficient but too slow for large areas. For a typical room, a 3/8 inch nap roller on smooth walls gives the best balance of coverage and material efficiency.
Sources: Paint Quality Institute, manufacturer technical data sheets (Sherwin-Williams, Behr, Benjamin Moore), and the Master Painters Institute architectural coating standards. Coverage figures reflect typical interior latex paint on primed surfaces; specialty coatings such as elastomerics and primers vary. The Metriova paint calculator performs the area, subtraction, and coat multiplication automatically from your room dimensions.
Paint Calculator — Gallons & Cost for Any Room
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